Film forming method, film forming system and recording medium

ABSTRACT

After silicon nitride films have been formed on wafers by a film forming process in a reaction vessel, the reaction vessel is processed by a purging process specified by a purging recipe and compatible with the film forming process to suppress production of gases and particles by removing surface parts of films deposited on the inside surface of the reaction vessel and causative of production of gases and particles. 
     A wafer boat  25  holding a plurality of wafers W is loaded into a reaction vessel  2 , and the wafers W are processed by a film forming process specified by a film forming recipe  1  specifying, for example, Si 2 Cl 2  gas and NH 3  gas as film forming gases. Subsequently, a purging recipe  1  specifying a purging process compatible to the film forming process is selected automatically, and the reaction vessel  2  is processed by the purging process specified by the purging recipe  1 . A purging recipe is selected automatically from a plurality of purging recipes specifying purging processes respectively compatible with film forming processes. Unnecessary extension of purging time is suppressed and the reaction vessel  2  can be processed by an appropriate purging process compatible with the film forming process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a film forming method including apurging process for purging a reaction vessel with a purging gas aftercompleting a film forming process for forming, for example, a siliconnitride film on a surface of a substrate, a film forming system forcarrying out the film forming method, and a recording medium storingprograms to be executed by a computer to carry out the film formingmethod.

2. Description of the Related Art

A semiconductor device fabricating process includes a film formingprocess for forming a silicon nitride film, namely, a film of Si₃N₄(hereinafter referred to as “SiN film”) on a substrate, such as asemiconductor wafer (hereinafter, referred to as “wafer”) W. The SiNfilm is formed on each of wafers W by a chemical vapor deposition method(CVD method) using a batch type thermal processing system. The SiN filmis formed a surface of each of the wafers W by carrying a wafer holderholding the wafers W in layers into a vertical reaction vessel ofquartz, keeping the interior of the reaction vessel at a predeterminedpressure and supplying process gases for depositing the SiN film intothe reaction vessel.

A main product and by-products of a film forming reaction deposit infilms on the inside surface of the reaction vessel and the wafer holderafter the thermal processing system has repeated the film formingprocess for forming the SiN film. The deposited films grow gradually asthe number of cycles of the film forming process increases. After thethickness of the deposited films has increased to a predeterminedthickness, the deposited films produce gases, the deposited films arecracked or the deposited films come off the surfaces in particles whenthe reaction vessel is heated for the next cycle of the film formingprocess.

To avoid such troubles, a cleaning gas is supplied into the reactionvessel to remove the deposited film deposited on the reaction vesselcompletely by the etching effect of the cleaning gas after the filmforming process has been repeated by a predetermined number of cycles.If the reaction vessel is cleaned by etching after every cycle of thefilm forming process, the reaction vessel of quartz is deteriorated andit takes a considerably long time to remove the cleaning gas completelyfrom the reaction vessel. Therefore, a purging process is executed afterthe completion of every cycle of the film forming process.

The purging process is carried out after the wafer holder holding theprocessed wafers W has been unloaded from the reaction vessel and beforethe wafer holder holding unprocessed wafers W is loaded into thereaction vessel. In the purging process, an empty wafer holder notholding any wafers W is carried into the reaction vessel, the interiorof the reaction vessel is kept at a predetermined pressure and at apredetermined temperature, a purging gas, such as nitrogen gas (N₂) issupplied into the reaction vessel, the reaction vessel is cooledrapidly, is evacuated and heated to remove surface layers of the filmsadhering to the inner surface of the reaction vessel positively by thepurging process to suppress the generation of gasses and the productionof particles. The purging process thus removes the surface layers of thedeposited films are about to come off the reaction vessel. Removal ofthe surface layers of the deposited film is effective in suppressing theproduction of gasses and particles in the film forming process to beexecuted subsequently to the purging process.

The purging process is controlled on the basis of a fixed purging reciperegardless of the type of the film forming process carried out in thereaction vessel and the thickness of the film formed of filmssuccessively deposited on the inside surface of the reaction vessel.Time necessary to complete the purging recipe is on the order of 50 min.The purging recipe is designed for the strongest purging process forremoving a very thick deposited film or a film of the type that isdifficult to remove. Therefore, the production of gases and particlesfrom most deposited films can be suppressed without continuing thepurging process for such a long time specified by the purging recipe andhence some part of the purging process is unnecessary. The time spentfor the unnecessary part of the purging process is a downtime of thefilm forming system. Increase in the downtime decreases the throughputof the film forming system. The purging process includes the steps ofcooling the interior of the reaction vessel at a predeterminedtemperature, heating the interior of the reaction vessel and evacuatingthe reaction vessel. Thus the unnecessary purging process increases theprocessing cost.

Such a problem may be solved by modifying the purging recipe for eachcycle of the film forming process taking into consideration theestimated amount of particles that may be produced in the reactionvessel. However, much time is needed for the analysis of the causes ofparticle production and, consequently, time necessary for the purgingprocess increases.

A technique for reducing the amount of particles adhering to the wafersW is proposed in Patent document 1. This previously proposed techniquecracks the deposited films adhering to the inside surface of a reactionfurnace by varying the temperature in the reaction furnace and makes apurging gas carry particles produced when the deposited films arecracked outside the reaction furnace. However, this previously proposedtechnique uses the same purging recipe regardless of the type of thefilm forming process and hence cannot solve the foregoing problem.

Patent document 1: JP 200-306904 A

SUMMARY OF THE INVENTION

The present invention has been made in view of such a circumstances andit is therefore an object of the present invention to provide atechnique capable of achieving an efficient purging process and ofsuppressing the production of gases and particles in a film formingprocess subsequent to the purging process by automatically selecting aproper purging recipe from a plurality of previously prepared purgingrecipes on the basis of the thickness of a thin film formed by aselected film forming recipe and processing a reaction vessel by apurging process specified by the selected purging recipe.

A film forming method of processing substrates by a film forming processusing film forming gases by a thermal processing system provided with aplurality of film forming recipes includes the steps of: processingsubstrates held on a substrate holder placed in a reaction vessel by afilm forming process specified by a selected one of the film formingrecipes; unloading the substrates from the reaction vessel;automatically selecting one of a plurality of previously preparedpurging recipes on the basis of a cumulative thickness equal to the sumof thicknesses of thin films formed by cycles of the film formingprocess specified by the selected film forming recipe; and carrying outa purging process specified by the selected purging recipe to purge thereaction vessel by supplying a purging gas into the reaction vesselafter the substrates have been unloaded from the reaction vessel.

The plurality of film forming recipes include those respectivelyspecifying different film forming gases to form thin films of the sametype, and those respectively specifying the same film forming gases anddifferent process temperature ranges. The plurality of purging recipesinclude those differing from each other in at least either of purgingtime and temperature condition in the reaction vessel. Differenttemperature conditions in the reaction vessel are specified by differenttemperature profiles, namely, different patterns of temperature changewith time.

The substrate holder holds a plurality of substrates in parallel layers,and the purging process includes a first purging process that is carriedout in a state where the reaction vessel is sealed after the substrateholder has been unloaded from the reaction vessel. The substrate holderholds a plurality of substrates in parallel layers, and the purgingprocess includes a second purging process that is carried out in a statewhere the empty substrate holder is placed in the sealed reactionvessel.

The film forming method according to the present invention may furtherinclude the step of changing the purging recipe on the basis of at leasteither of the measured number of particles in the reaction vessel andthe measured number of particles adhering to the surfaces of thesubstrates. In the film forming method according to the presentinvention, the first and/or the second pursing process may include thestep of forcibly cooling the reaction vessel and the second purgingprocess may include the step of heating the reaction vessel at atemperature higher than a process temperature for a film forming processfollowing the purging process.

A film forming system that forms films on substrates held on a substrateholder placed in a reaction vessel by a film forming process specifiedby a film forming recipe selected from a plurality of different filmforming recipes and carries out a purging process by supplying a purginggas into the reaction vessel after the substrates have been unloadedfrom the reaction vessel includes: a storage unit storing a plurality ofpurging recipes respectively specifying different purging processes;managing means for managing a cumulative thickness equal to the sum ofthicknesses of thin films formed by cycles of a film forming processspecified by each of the film forming recipes; and a purging recipeselecting means for selecting a purging recipe from the plurality ofpurging recipes on the basis of the cumulative thickness equal to thesum of the thicknesses of thin films formed by a film forming processspecified by the selected film forming recipe.

The plurality of film forming recipes include those respectivelyspecifying different film forming gases for forming the same thin filmand those respectively specifying different process temperature rangesand the same film forming gases. The plurality of purging recipesinclude those differing from each other in at least either process timefor the purging process and temperature in the reaction vessel.

In the film forming system according to the present invention, thesubstrate holder holds a plurality of substrates in parallel layers, andthe purging process may include a first purging process that is carriedout in a state where the reaction vessel is sealed after the substrateholder has been unloaded from the reaction vessel. In the film formingsystem according to the present invention, the substrate holder holds aplurality of substrates in parallel layers, and the purging process mayinclude a second purging process that is carried out in a state wherethe empty substrate holder is placed in the sealed reaction vessel.

A recording medium according to the present invention stores programs tobe executed by a computer to carry out the film forming method accordingto the present invention. More concretely, the programs to be executedby the computer specifies the steps of: processing substrates held on asubstrate holder placed in a reaction vessel by a film forming processon the basis of a selected film forming recipe; unloading the substratesfrom the reaction vessel; automatically selecting one of a plurality ofpreviously prepared purging recipes on the basis of a cumulativethickness equal to the sum of thicknesses of thin films formed by thefilm forming process based on the selected film forming recipe; andcarrying out a purging process by supplying a purging gas into thereaction vessel on the basis of the selected purging recipe. Thecumulative thickness is the sum of the thicknesses of thin films formedsince starting using a new reaction vessel or since resuming using thereaction vessel after cleaning the interior thereof. For example, thecumulative thickness is calculated by adding up desired film thicknessesrespectively for film forming processes.

The present invention selects the purging recipe on the basis of thecumulative thickness equal to the sum of the thicknesses of thin filmsformed by a film forming process specified by the selected film formingrecipe. The cumulative thickness is not necessarily for only each of thefilm forming recipes. According to the present invention, the cumulativethickness may be for a group of a plurality of film forming recipes, andthe purging recipe may be selected on the basis of the cumulativethickness for the group of film forming recipes.

The recording medium is any one of flexible disks, compact disks andmagnetooptical disks, namely, the so-called MO disks.

According to the present invention, the purging recipe is selectedautomatically from the plurality of previously prepared purging recipeson the basis of the cumulative thickness equal to the sum of thin filmsformed by a selected film forming recipe, and the reaction vessel ispurged by a purging process specified by the selected purging recipe.Thus a proper purging process compatible with the film forming processis executed. Some film forming processes specified by some film formingrecipes require a purging process specified by a purging recipe having astrong purging effect and some other film forming processes do notrequire a purging process specified by a purging recipe having a strongpurging effect. The purging process having a strong purging effect takesa long process time. The proper purging process compatible with the filmforming recipe reduces wasteful process time for the purging process,enables removing films deposited on the inside surface of the reactionvessel and being causative of the production of gases and particles, andcan suppress the production gases and particles in the film formingprocess to be carried out subsequently to the purging process.

Consequently, the downtime for which the film forming system isinoperative is eliminated or reduced to the least possible extent, andthe reduction of the through put of the film forming system due to thepurging process can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a film forming system forcarrying out a film forming method in a preferred embodiment accordingto the present invention;

FIG. 2 is a block diagram of a control unit included in the film formingsystem shown in FIG. 1;

FIG. 3 is a diagram showing the characteristics of a purging recipe 1specifying a purging process to be carried out by the film formingsystem shown in FIG. 1;

FIG. 4 is a diagram showing the characteristics of a purging recipe 2specifying a purging process to be carried out by the film formingsystem shown in FIG. 1;

FIG. 5 is a diagram showing the characteristics of a purging recipe 3specifying a purging process to be carried out by the film formingsystem shown in FIG. 1;

FIG. 6 is a diagram showing the characteristics of a purging recipe 4specifying a purging process to be carried out by the film formingsystem shown in FIG. 1;

FIG. 7 is a table showing process recipes specifying processes to becarried out by the film forming system shown in FIG. 1;

FIG. 8 is a flow chart of a process recipe 1 specifying conditions for afirst process to be executed by the film forming system shown in FIG. 1;

FIG. 9 is a flow chart of a process recipe 2 specifying conditions for asecond process to be executed by the film forming system shown in FIG.1;

FIG. 10 is a flow chart of a process recipe 3 specifying conditions fora third process to be executed by the film forming system shown in FIG.1;

FIG. 11 is a flow chart of a process recipe 4 specifying conditions fora fourth process to be executed by the film forming system shown in FIG.1;

FIG. 12 is a pictorial view of assistance in explaining a first purgingprocess and a second purging process to be executed by the film formingsystem shown in FIG. 1; and

FIG. 13 is a typical side elevation of assistance in explaining apeeling mode in which a SiN film deposited on the inside surface of aprocessing vessel comes off the inside surface of the processing vessel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, a film forming system in a first embodiment according to thepresent invention for carrying out a film forming method according tothe present invention will be described. In FIG. 1 showing a filmforming system in a batch type low-pressure CVD system, indicated at 2is a cylindrical, vertical reaction vessel made of, for example, quartz.The reaction vessel 2 has an open lower end 21 serving as a furnaceopening. A flange 22 is formed integrally with the reaction vessel 2around the open lower end 21. A first lid 23 made of, for example,quartz is dispose below the reaction vessel 2. The first lid 23 is movedvertically by a boat elevator 20 to bring the first lid 23 into closecontact with the open lower end 21 to seal the reaction vessel 2 and toseparate the first lid 23 from the open lower end 21 to open thereaction vessel 2. The boat elevator 20 is moved vertically by a liftingmechanism 20 a. A rotating shaft 24 is extended vertically through acentral part of the first lid 23. A wafer boat 25, namely, a substrateholder, is mounted on top of the rotating shaft 24.

The wafer boat 25 is provided with three or more upright support bars26. In this embodiment, the wafer boat 25 is provided with four supportbars 26. The support bars 26 are provided with slots to support, forexample, one hundred and twenty-five wafers W, namely, workpieces, in avertical arrangement on the wafer boat 25. Some dummy wafers arearranged in upper and lower end parts of a wafer holding range in whichthe wafers W are held and the wafers W to be processed are arranged in amiddle part of the wafer holding range between the upper and the lowerend part. A motor M for driving the rotating shaft 25 for rotation isdisposed below the lower end of the rotating shaft 25. Thus the waferboat 25 is rotated by the motor M. A heat insulating unit 27 is mountedon the first lid 23 so as to surround the rotating shaft 24.

The boat elevator 20 moves the wafer boat 25 vertically between aprocessing position in the reaction vessel 2 and a loading position in aloading area 28 where the wafers W are loaded into and unloaded from thewafer boat 25. When the boat elevator 20 positions the wafer boat 25 atthe processing position, the first lid 23 closes the open lower end 21of the reaction vessel 2. A second lid 29 made of, for example, quartzis disposed below the reaction vessel 2. The second lid 29 can behorizontally moved by a driving mechanism 29 a. When the first lid 29 isheld in the loading area 28 and the open lower end 21 is opened, thesecond lid 29 covers the open lower end 21 of the reaction vessel 2closely to seal the reaction vessel 2.

An L-shaped injector 31 is passed through a hole formed in the flange 22of the reaction vessel 2 into the reaction vessel 2 to supply a gas intothe reaction vessel 2 holding the wafers W therein. A gas supply pipe32, namely, a gas supply line, has one end connected to a base end ofthe injector 31 and the other end connected through a gas supply controlunit 100 to, for example, four film forming gas sources 33, 34, 35 and36 and a purging gas source 37. The film forming gas needed for filmformation is injected by the injector 31 into the reaction vessel 2. Thegas supply control unit 100 has flow control devices including valves V1to V5 and flow regulators M1 to M5.

In this embodiment, the film forming gas sources 33, 34, 35 and 36supply SiH₂Cl₂ gas (dichlorosilane gas, abbreviated to DCS gas), Si₂Cl₆gas (HCD gas), bis-tertiary butyl aminosilane gas (BTBAS gas) andammonia gas (NH₃ gas), respectively. The purging gas source supplies aninert gas, such as nitrogen gas (N₂). The purging gas is not limited toan inert gas.

A discharge opening for discharging gases from the reaction vessel 2 isformed in an upper part of the reaction vessel 2. A discharge pipe 43 isconnected to the discharge opening. A vacuum pump 41, namely, anevacuating means, for evacuating the reaction vessel 2 to create adesired vacuum in the reaction vessel 2, and a pressure regulator 42,such as a butterfly valve, are placed in the discharge pipe 43. A mainline 45 provided with a main valve MV, and a by-pass line 46 provided,for example, a needle valve NV and connected in parallel to the mainline 45 are placed in a part of the discharge pipe 43 between the vacuumpump 41 and the pressure regulator 42. The main valve MV is closed andthe needle valve NV is opened to discharge gases from the reactionvessel 2 through the by-pass line 46 at a low discharge rate of, forexample, about 30 l/min. Both the main valve MV and the needle valve NVare opened to evacuate the reaction vessel 2 through both the main line45 and the by-pass line 46. A discharge control unit 200 is representedby an area surrounded by chain lines in FIG. 1.

The reaction vessel 2 is placed in a heating furnace 52. The heatingfurnace 52 is provided with a heater 51, namely, a heating means, forheating the interior of the reaction vessel 2. Preferably, the heater 51includes a carbon wire or the like. The carbon wire does not causecontamination and has an excellent heating characteristic. A powercontroller 51 a controls the power supplied to the heater 51. An annularair supply duct 53 is disposed in an annular space between the reactionvessel 2 and the heating furnace 52 so as to surround a lower part ofthe reaction vessel 2. The air supply duct 53 is provided in its upperwall with a plurality of outlets 55. An air supply pipe 54 for carryingcooling air cooled at a temperature on the order of 0° C. is connectedto the air supply duct 53. Cooled air is supplied through the air supplypipe 54 and the air supply duct 53 for forced cooling. Shown also inFIG. 1 are a fan 56 for blowing air into the air supply duct 53, adischarge line 57 and an air supply system 58 including a valve and aflow regulator.

The film forming system is provided with a first particle detector 61,placed in the loading area 28, and a second particle detector 62 placedin the discharge pipe 43. The first particle detector 61 and the secondparticle detector 62 give detection signals to a controller. The firstparticle detector 61 is a particle counter for counting particles on thesurfaces of wafers held outside or inside the film forming system. Thesecond particle detector 62 is a particle counter for counting particlesin a gas phase prevailing in the reaction vessel 2 or a discharge systemincluding the discharge pipe 43.

The film forming system is provided with a controller 7 including acomputer. The controller 7 controls the operations of the liftingmechanism 20 a of the boat elevator 20, the driving mechanism 29 a fordriving the second lid 29, the power controller 51 a for controlling thepower supplied to the heater 51, the gas supply control unit 100, thedischarge control unit 200 and the air supply system 58. Actually, thecontroller 7 includes a CPU (central processing unit), programs storedin a storage device and storage devices. FIG. 2 shows the controller 7in a block diagram. The present invention is featured by a purgingprocess for purging the reaction vessel 2 of the film forming system andhence priority will be given to the explanation of components relatedwith the purging process.

Referring to FIG. 2, connected to a bus 70 are a film forming recipestorage device 71, a purging recipe storage device 72, a purging recipeselecting program storage device 73, a film thickness managing unit 74,an operating time managing unit 75 for managing the operating time ofthe reaction vessel 2, a particle number managing unit 76, the firstparticle detector 61, the second particle detector 62, the air supplycontrol unit 100, the discharge control unit 200, the air supply system58, the power control unit 51 a for controlling power supplied to theheater 51, the lifting mechanism 20 a of the boat elevator 20 and thedriving mechanism 29 a for driving the second lid 29.

The film forming recipe storage device 71 stores a plurality of filmforming recipes including those for forming the same thin filmrespectively using different film forming gases and those for formingthe same film using the same film forming gas and respectively usingprocess temperatures in different temperature ranges.

This embodiment prepares four recipes, namely, a film forming recipe 1,a film forming recipe 2, a film forming recipe 3 and a film formingrecipe 4. The film forming recipe specifies DCS gas as the film forminggas. The film forming recipe 2 specifies HCD gas as the film forming gasand a process temperature below 500° C. The film forming recipe 3specifies HCD gas as the film forming gas and process temperatures notlower than 500° C. The film forming recipe 4 specifies BTBAS gas as thefilm forming gas.

The purging recipe storage device 72, namely, the storage device, storesa plurality of purging recipes. The purging recipes describe conditionsfor purging processes. The purging process is carried out in a periodbetween successive film forming processes to purge the reaction vesselwith a purging gas. The plurality of purging recipes include thosespecifying different values for at least either of the process time ofthe purging process and temperature of the interior of the reactionvessel. Difference in the temperature of the interior of the reactionvessel signifies difference in temperature profile representing thevariation of set temperature with time.

FIGS. 3, 4, 5 and 6 shows purging recipes 1, 2, 3 and 4 respectivelycorresponding to the film forming recipes 1, 2, 3 and 4, respectively,by way of example.

The purging recipe selecting program 73 is a purging recipe selectingmeans for selecting one of the plurality of purging recipes on the basisof a cumulative thickness equal to the sum of the thicknesses of thinfilms formed by cycles of a film forming process specified by a selectedfilm forming recipe. More concretely, a predetermined purging recipe isselected from the plurality of previously prepared purging recipes onthe basis of the film forming recipe and the cumulative thickness. FIGS.8 to 11 show flow charts of purging recipe selecting procedures forselecting the purging recipes to be used respectively in combinationwith the selected film forming recipes, respectively. A predeterminedpurging recipe is selected by the purging recipe selecting procedure.FIG. 7 is a table for facilitating understanding the relation betweenthe film forming recipes and the purging recipes.

The film thickness managing unit 74 obtains the cumulative thicknessequal to the sum of the thicknesses of thin films formed by the selectedfilm forming recipe. The cumulative thickness is the sum of thethicknesses of thin films formed since starting using a new reactionvessel or since resuming using the reaction vessel after cleaning theinterior thereof. For example, the cumulative thickness is calculated byadding up desired film thicknesses respectively for film formingprocesses.

The operating time managing unit 75 calculates a cumulative operatingtime for which the reaction vessel 2 has been operated for cycles of afilm forming process specified by the film forming recipe. The operatingtime is a time between a moment the open lower end of the reactionvessel 2 is closed by the first lid 23 after the wafer boat 25 holdingunprocessed wafer W has been loaded into the reaction vessel 2 and amoment the open lower end of the reaction vessel 2 is closed by thesecond lid 29 after the wafer boat 25 holding the processed wafers W hasbeen unloaded from the reaction vessel 2.

The particle number managing unit 76 receives data on the measurednumbers of particles from the first particle detector 61 and a secondparticle detector 62. The particle number managing unit 76 provides anabnormal particle production signal at a predetermined time when themode of production of particles changes, for example, when the meanparticle number is doubled or tripled.

The software stored in the controller 7, namely, the process programs,the film forming recipes, purging recipes and graphic data for enteringthe film forming recipes and particle recipes, is recorded in arecording medium, such as a flexible disk, a compact disk, amagnetooptical disk, namely, the so-called MO disk, installed in thecontroller 7. The process programs are computer programs specifyingoperations to be carried out by the film forming system. The purgingrecipe selecting program is one of the process programs.

A film forming method to be carried out by the film forming system willbe described by way of example. The film forming method will bedescribed with reference to a flow chart shown in FIG. 8 on anassumption that DCS gas is used as the film forming gas for forming aSiN film. Execution of steps of the procedure represented by the flowchart shown in FIG. 8 is controlled by the computer program. The filmforming recipe 1 is selected in step SI. The cumulative thickness andthe cumulative operating time for which the reaction vessel 2 has beenused are calculated, particles are counted and the film forming processspecified by the film forming recipe 1 is carried out in step S2. Apredetermined number of wafers W are mounted on the wafer boat 25 andthe wafer boat 25 is loaded into the reaction vessel 2 by lifting up thefirst lid 23 by the boat elevator 20.

After the wafer boat 25 has been loaded into the reaction vessel 2 andthe open lower end 21 of the reaction vessel 2 has been closed by thefirst lid 23, the interior of the reaction vessel 2 is heated at atemperature in the range of, for example, 650° C. to 760° C., the mainvalve MV and the needle valve NV are opened and the vacuum pump 41 isoperated to evacuate the reaction vessel 2 to a predetermined vacuum of,for example, 0.4 Pa.

Then, DCS gas and NH₃ gas are supplied from the first film forming gassource 33 and the second film forming gas source 34 at predeterminedflow rates, respectively, into the reaction vessel 2 to carry out thefilm forming process for forming a SiN film. The DCS gas and the NH₃ gassupplied into the reaction vessel 2 are decomposed by thermaldecomposition and a SiN film is deposited on the surface of each waferW. During those steps of the film forming process, the motor M rotatesthe wafer boat 25. The supply of DCS gas and NH₃ gas is stopped, theatmosphere in the reaction vessel is discharged from the reaction vessel2, N₂, namely, a purging gas, is supplied for purging from the purginggas source 37 into the reaction vessel 2. After the reaction vessel 2has been thus purged of the atmosphere, the pressure of the interior ofthe reaction vessel 2 is adjusted to the atmospheric pressure. Then, thewafer boat 25 is unloaded from the reaction vessel 2 and the filmforming process specified by the film forming recipe 1 is terminated instep S3.

Subsequently, the purging process is carried out. First the purgingrecipe is selected. A query is made in step S4 to see if the particlenumber managing unit 76 decided that the number of particles is normal.If the number of particles is normal, the film forming method goes tostep S5. If the number of particles is abnormal the purging processspecified by the purging recipe 4 is executed in step S6.

A query is made in step S5 to see if the cumulative thickness calculatedby the film thickness managing unit 74 is greater than a thresholdcumulative thickness A of, for example, 1.5 μm. If the response to thequery in step S5 is negative, the purging process specified by thepurging recipe 1 is carried out in step S7. If the response to the queryin step S5 is affirmative, the purging process specified by the purgingrecipe 4 is carried out in step S6. After the purging process specifiedby the selected purging recipe has been completed, the wafer boat 25holding unprocessed wafers W is loaded into the reaction vessel 2 andthen the next cycle of the film forming process is started in step S8.

The film forming method will be described with reference to a flow chartshown in FIG. 9 on an assumption that HCD gas is used as the filmforming gas for forming a SiN film at a process temperature below 500°C. The film forming recipe 2 is selected in step S21. The cumulativethickness and the cumulative operating time for which the reactionvessel 2 has been used are calculated, particles are counted and thefilm forming process specified by the film forming recipe 2 is carriedout in step S22. HCD gas and NH₃ gas are used as film forming gases, theinterior of the reaction vessel 2 is heated at a temperature not lowerthan 450° C. and below 500° C. The film forming process specified by thefilm forming recipe 2 is terminated in step S23.

Subsequently a purging process is started. A query is made in step S24to see if the number of particles is normal. Step S25 is executed if theresponse to the query in step S24 is affirmative. Step S29 is executedto carry out the purging process specified by the purging recipe 4 ifthe response to the query in step S24 is negative.

A query is made in step S25 to see if the cumulative thicknesscalculated by the film thickness managing unit 74 is greater than asecond threshold cumulative thickness A2 of, for example, 5000 Å. StepS26 is executed if the response to the query in step S25 is negative.Step S29 is executed to carry out the purging process specified by thepurging recipe 4 if the response to the query in step S25 isaffirmative.

A query is made in step S26 to see if the cumulative thicknesscalculated by the film thickness managing unit 74 is greater than afirst threshold cumulative thickness Al of, for example, 1000 Å. Thepurging process specified by the purging recipe 2 is carried out in stepS27 if the response to the query in step S26 is negative. The purgingprocess specified by the purging recipe 3 is carried out in step S28 ifthe response to the query in step S26 is affirmative. After the purgingprocess specified by the selected purging recipe has been completed, thenext cycle of the film forming process is started in step S30.

The film forming method will be described with reference to a flow chartshown in FIG. 10 on an assumption that HCD gas is used as the filmforming gas for forming a SiN film at a process temperature not lowerthan 500° C. The film forming recipe 3 is selected in step S41. Thecumulative thickness and the cumulative operating time for which thereaction vessel 2 has been used are calculated, particles are countedand the film forming process specified by the film forming recipe 3 iscarried out in step S42. HCD gas and NH₃ gas are used as film forminggases, the interior of the reaction vessel 2 is heated at a temperaturein the range of 500° C. and 600° C. The film forming process specifiedby the film forming recipe 3 is terminated in step S43.

Subsequently a purging process is started. A query is made in step S44to see if the number of particles is normal. Step S45 is executed if theresponse to the query in step S44 is affirmative. Step S47 is executedto carry out the purging process specified by the purging recipe 4 ifthe response to the query in step S44 is negative.

A query is made in step S45 to see if the cumulative thicknesscalculated by the film thickness managing unit 74 is greater than athreshold cumulative thickness A of, for example, 5000 Å. Step S46 isexecuted if the response to the query in step S45 is negative. Step S47is executed to carry out the purging process specified by the purgingrecipe 4 if the response to the query in step S45 is affirmative. Afterthe purging process specified by the selected purging recipe has beencompleted, the next cycle of the film forming process is started in stepS48.

The film forming method will be described with reference to a flow chartshown in FIG. 11 on an assumption that BTBAS gas is used as the filmforming gas for forming a SiN film. The film forming recipe 4 isselected in step S51. The film forming process specified by the filmforming recipe 4 is carried out in step S52. BTBAS gas and NH₃ gas areused as film forming gases and the interior of the reaction vessel 2 isheated at a temperature in the range of, for example 450° C. and 600° C.After the film forming process specified by the film forming recipe 4has been terminated in step S53, the purging recipe 4 is selected andthe purging process specified by the purging recipe 4 is carried out instep S54. After the purging process has been completed, the next cycleof the film forming process is started in step S55.

Those purging recipes will be described. Referring to FIG. 3, thepurging process specified by the purging recipe 1 is started at time T1when the wafer boat 25 is carried to the loading area 28 and the secondlid 29 closes the open lower end 21 of the reaction vessel 2. Upon thestart of the purging process, supply of power to the heater 51 isstopped and the reaction vessel 2 is cooled in a forced cooling mode bysupplying cold air of, for example 0° C. through the air supply ductinto the space between the reaction vessel 2 and the heating furnace 52and discharging air from the space through the discharge line 57. Thusthe reaction vessel 2 is cooled rapidly to a first temperature of, forexample, 300° C. by time T2. Then, supply of the cold air is stopped andsupply of power to the heater 51 is resumed to heat the reaction vessel2, for example, at the temperature for the film forming process in therange of 650° C. to 760° C. by time T3. The first temperature and timeT2 are determined such that the reaction vessel 2 is cooled rapidly at acooling rate of, for example, 75° C./min so that the difference betweenthe temperature of the reaction vessel 2 at the start of the purgingprocess and that of the reaction vessel 2 at time T2 is in the range ofabout 100° C. to about 500° C.

In a period between times T1 and T3, the purging gas is supplied fromthe purging gas source 37 through the gas supply pipe 32 into thereaction vessel 2 at a flow rate, for example, on the order of 30 slm,the main valve MV is closed, the needle valve NV is opened and thevacuum pump 41 is operated to discharge the atmosphere in the reactionvessel 2 through the by-pass line 46 at a discharge rate, for example,on the order of 30 l/min. The purging process specified by the purgingrecipe 1 is a first purging process that is carried out without placingthe wafer boat 25 in the reaction vessel 2 as shown in FIG. 12. Processtime needed for the first purging process is, for example, on the orderof 12.5 min.

The purging recipe 1 is a basic recipe for the film forming processspecified by the film forming recipe 1 specifying DCS gas as a filmforming gas. Production of particles due to generation of gases andseparation of films deposited on the inside surface of the reactionvessel 2 can be suppressed by carrying out the purging process specifiedby the purging recipe 1 after the completion of every cycle of the filmforming process specified by the film forming recipe 1, provided thatpredetermined conditions are satisfied, i.e., provided that the numberof particles is normal, the cumulative thickness is below the thresholdcumulative thickness A and the cumulative operating time is below alimit cumulative operating time B.

When the reaction vessel 2 is cooled rapidly by forced cooling, films ofa reaction main product and reaction by-products crack due to differencein heat capacity between those films and the reaction vessel 2 made ofquartz.

When the atmosphere in the reaction vessel 2 is discharged at thedischarge rate on the order of 30 l/min, surface parts of the depositedfilms that are about to come off are removed and are discharge outsidethe reaction vessel 2. Consequently, production of gases and particlesdue to the separation of the surface parts of the deposited films thatare about to come off can be suppressed and the production of gases andparticles in the film forming process following the purging process canbe prevented. If undesired gasses are produced by the deposited filmsduring the film forming process, the strength of adhesion of thedeposited films to the inside surface of the reaction vessel is reducedand the deposited films are liable to come off the reaction vessel 2.Therefore, the production of gases needs to be suppressed.

The open lower end 21 of the reaction vessel 2 is closed by the secondlid 29 during the purging process. Therefore, the wafer boat 25 held inthe loading area 28 will not be contaminated with the films peed off theinside surface of the reaction vessel 2. Purging process time specifiedby the purging recipe 1 is 12.5 min, which is a reloading time necessaryfor unloading the processed wafers W from the wafer boat 25 and loadingunprocessed wafers W on the wafer boat 25 in the loading area 28.Therefore, down time for which the film forming system is not operateddoes not increase because the purging process does not need any time forthe purging process and the through put of the film forming system doesnot decrease when the purging process is carried out during thereloading time.

A purging process 2 specified by the purging recipe 2 will be describedwith reference to FIG. 4. The purging process 2 is carried out with theempty wafer boat 25 placed in the reaction vessel 2. The wafer boat 25holding the processed wafers W is unloaded from the reaction vessel 2,the processed wafers are unloaded from the wafer boat 25, the emptywafer boat 25 is placed in the reaction vessel 2, and then the purgingprocess 2 is started.

The purging process 2 is started when the open lower end 21 of thereaction vessel 2 is closed by the first lid 23 after the empty waferboat 25 has been loaded into the reaction vessel 2. The temperature ofthe interior of the reaction vessel 2 when the empty wafer boat 25 isloaded into the reaction vessel 2 is, for example, equal to thetemperature for the film forming process between 450° C. and atemperature below 500° C. In a waiting period subsequent to a period forloading the empty wafer boat 25 into the reaction vessel 2, N₂ issupplied from the purging gas source 37 through the gas supply pipe 32into the reaction vessel 2 at a flow rate on the order of 1 slm.

Then, the reaction vessel 2 is evacuated. The vacuum pump 41 is operatedand the needle valve NV is opened to discharge the atmosphere in thereaction vessel 2 through the by-pass line 46. Then, the main valve MVis opened to discharge the atmosphere in the reaction vessel 2 throughthe main line 45 to evacuate the reaction vessel to a vacuum on theorder of 0.4 Pa. In this example, the pressure in the evacuated reactionvessel 2 is in the range of about 0.4 to about 1.0 Pa.

Supply of power to the heater 51 is started at time T4 to heat theinterior of the reaction vessel 2 at a second temperature of, forexample, 850° C. by time T5 during a purging cycle using N₂. Desirably,the second temperature is higher than a process temperature for the filmforming process to be carried out after the purging process. Cooling thereaction vessel 2 is started at time T6 to cool the reaction vessel 2 toa temperature, for example, equal to the process temperature for thefilm forming process between 450° C. and a temperature below 500° C. bytime T7.

In the purging cycle, N₂ is supplied from the purging gas source 37through the gas supply pipe 32 into the reaction vessel 2 at a flow rateof 1 slm to purge the reaction vessel 2 of the atmosphere with N₂ (N₂purging step), supply of N₂ is stopped, and then the vacuum pump 41 isoperated to evacuate the reaction vessel 2 (VAC step). The vacuum pump41 is operated and the main valve MV is opened to discharge theatmosphere in the reaction vessel 2 while N₂ gas is being supplied intothe reaction vessel 2.

Purging and discharging are repeated alternately. Supply of N₂ isstopped after purging has been repeated three times, and then the mainvalve MV and the needle valve NV are closed and the vacuum pump 41 isstopped after discharging has been repeated three times. Then, N₂ issupplied from the purging gas source 37 into the reaction vessel 2 at aflow rate of 30 slm to adjust the pressure in the reaction vessel 2 tothe atmospheric pressure. Then, N₂ is supplied from the purging gassource 37 into the reaction vessel 2 at 1 slm and the wafer boat 25 isunloaded from the reaction vessel to complete the purging process. Theprocess time of the purging process is, for example, on the order of30.7 min.

The purging process specified by the purging recipe 2 is effective insuppressing the production of particles caused by degassing and theresultant separation of the deposited film from the inside surface ofthe reaction vessel 2. The purging recipe 2 is a basic recipe specifyinga film forming process using HCD gas specified by the film formingrecipe 2 as a film forming gas and a temperature below 500° C. Theforegoing predetermined conditions can suppress production of particlesdue to production of gases in the reaction vessel 2 and the resultantseparation of the deposited films from the inside surface of thereaction vessel 2 by carrying out the purging process specified by thepurging recipe 2 after the completion of every cycle of the film formingprocess specified by the film forming recipe 2.

Evacuation and heating at the second temperature of the reaction vesselare carried out simultaneously during the purging process to promote theseparation of the deposited film from the inside surface of the reactionvessel 2 and the resultant production of gases by the difference in heatcapacity between the reaction vessel 2 and the deposited films.Consequently, the remaining films are forced to produce gases. Heatingthe reaction vessel 2 at the second temperature higher than a processtemperature for the following film forming process suppresses theproduction of gasses during the film forming process.

The purging process is carried out with the empty wafer boat 25 placedin the reaction vessel 2. Thus the films deposited on the wafer boat 25can be removed to suppress production of particles and degassing of theresidual films can be suppressed.

A purging process specified by the purging recipe 3 will be describedwith reference to FIG. 5. The purging process specified by the purgingrecipe 3 is similar to the purging process 2 specified by the purgingrecipe 2. The purging process is carried out with the empty wafer boat25 placed in the reaction vessel 2. The wafer boat 25 holding theprocessed wafers W is unloaded from the reaction vessel 2, the processedwafers are unloaded from the wafer boat 25, the empty wafer boat 25 isplaced in the reaction vessel 2, and then the purging process specifiedby the purging recipe 3 is started.

The purging process is started when the open lower end 21 of thereaction vessel 2 is closed by the first lid 23 after the empty waferboat 25 has been loaded into the reaction vessel 2. In a waiting periodsubsequent to a period for loading the empty wafer boat 25 into thereaction vessel 2, N₂ is supplied from the purging gas source 37 throughthe gas supply pipe 32 into the reaction vessel 2 at a flow rate on theorder of 1 slm.

Supply of power to the heater 51 is stopped at time T8 and the reactionvessel 2 is cooled in a forced cooling mode by supplying cold air of,for example 0° C. through the air supply duct into the space between thereaction vessel 2 and the heating furnace 52 and discharging air fromthe space through the discharge line 57. Thus the reaction vessel 2 iscooled rapidly to a third temperature of, for example, 300° C. by timeT9. The third temperature is determined such that the difference betweenthe temperature of the reaction vessel at the start of the purgingprocess and that of the reaction vessel 2 at time T9 is in the range ofabout 100° C. to 500° C. Rapid cooling cools the reaction vessel 2 at acooling rate of, for example, 75° C./min.

Then, the reaction vessel 2 is evacuated. The vacuum pump 41 is operatedand the needle valve NV is opened to discharge the atmosphere in thereaction vessel 2 through the by-pass line 46. Then, the main valve MVis opened to discharge the atmosphere in the reaction vessel 2 throughthe main line 45 to evacuate the reaction vessel to a vacuum on theorder of 0.4 Pa. In this example, the pressure in the evacuated reactionvessel 2 is in the range of about 0.4 to about 1.0 Pa.

Subsequently, supply of cold air is stopped and supply of power to theheater 51 is resumed after the interior of the reaction vessel 2 hasbeen cooled to the third temperature by time T9. The reaction vessel 2is heated so that the interior of the reaction vessel 2 is heated at afourth temperature by time T10 and N₂ is supplied into the reactionvessel 2 for a purging cycle. Desirably, the fourth temperature ishigher than a process temperature for the following film forming processto be carried out subsequently to the purging process. The interior ofthe reaction vessel 2 is maintained at the fourth temperature until thewafer boat 25 is unloaded from the reaction vessel 2.

The purging cycle supplies N₂ from the purging gas source 37 through thegas supply pipe 32 into the reaction vessel 2 at a flow rate of 1 slm(N₂ purging step), stops supplying N₂ into the reaction vessel 2, andthen the reaction vessel 2 is evacuated by the vacuum pump 41 (VACstep). The main valve MV is opened during the N₂ purging step forevacuation using the vacuum pump 41. The N₂ purging step and the VACstep are repeated alternately. Nitrogen gas N₂ is supplied at 1 slm fromthe purging gas source 37 in the third cycle of the N₂ purging step.Then, supply of N₂ is stopped, the main valve MV and the needle valve NVare closed to stop evacuation using the vacuum pump 41, and N₂ issupplied from the purging gas source 37 at 30 slm to adjust the pressureof the interior of the reaction vessel 2 to the atmospheric pressure.Then, N₂ is supplied from the purging gas source 37 into the reactionvessel 2 at a flow rate of 2 slm and the wafer boat 25 is unloaded fromthe reaction vessel 2 to complete the purging process. The process timeof the purging process is, for example, on the order of 34.4 min.

The purging recipe 3 is a basic recipe used for the film forming processthat uses HCD gas specified by the film forming recipe 2 as the filmforming gas and is carried out at a temperature in the range of 500° C.to 600° C. The foregoing predetermined conditions can suppressproduction of particles due to production of gases in the reactionvessel 2 and the resultant separation of the deposited films from theinside surface of the reaction vessel 2 by carrying out the purgingprocess specified by the purging recipe 3 after the completion of everycycle of the film forming process specified by the film forming recipe3.

The purging effect of the purging process specified by the purgingrecipe 3 is higher than those of the purging processes specified by thepurging recipes 1 and 2, and the effect of the purging process specifiedby the purging recipe 3 in suppressing the production of gases and theresultant production of particles is higher than that of the purgingprocess specified by the purging recipe 1. Therefore, the production ofgases and particles during the film forming process specified by thefilm forming recipe 1 can be suppressed by carrying out the purgingprocess specified by the purging recipe 3 even if the cumulativethickness is greater than a predetermined threshold cumulative thicknessand the films deposited on the inside surface of the reaction vessel arecracked and ready to produced particles.

Since the effect of the purging recipe 3 in suppressing the productionof particles is higher than that of the purging recipe 2, production ofparticles can be suppressed during the film forming process specified bythe film forming recipe 2 by carrying out the purging process specifiedby the purging recipe 3 even if the cumulative thickness is greater thanthe first threshold cumulative thickness A1 of 1000 Å.

This purging process cools the reaction vessel 2 by forced air coolingto remove the surface layers, which are about to come off, of films of amain product and by-products that are deposited on the inside surface ofthe reaction vessel 2 and the discharges the removed surface layers ofthe films outside the reaction vessel 2 by discharging the atmosphere inthe reaction vessel 2 by the vacuum pump 41. Evacuation of the reactionvessel 2 promotes the separation of the deposited films from thereaction vessel 2. The forced air cooling of the reaction vessel 2during evacuation is more effective than the forced air cooling of thereaction vessel 2 at a normal pressure in removing the deposited filmfrom the reaction vessel 2. Removal of the surface layers of thedeposited film is effective in suppressing the production of particlesdue to the separation of the deposited films from the reaction vessel 2.

Surface layers of the films deposited on the inside surface of thereaction vessel 2 are made to become easy to peel off by evacuation andthe difference in heat capacity between the films and the reactionvessel when the reaction vessel 2 is heated at the fourth temperatureagain during evacuation. The residual films remaining on the insidesurface of the reaction vessel 2 after the surface layers have beenremoved are forced to produce gases. Heating the interior of thereaction vessel 2 at the fourth temperature higher than a processtemperature for the film forming process following the purging processis effective in suppressing production of gases in the film formingprocess.

The purging process is carried out with the empty wafer boat 25 placedin the reaction vessel 2. Thus the films deposited on the wafer boat 25can be removed to suppress production of particles and degassing of theresidual films can be suppressed.

A purging process specified by the purging recipe 4 will be describedwith reference to FIG. 6. The purging recipe 4 corresponds to aconventional purging recipe. The purging process specified by thepurging recipe 4 includes the first purging process and the secondpurging process which are carried out successively. The wafer boat 25 isunloaded from the reaction vessel 2 and is carried to the unloading area28, the open lower end 21 of the reaction vessel 2 is closed by thesecond lid 29, and then the purging process specified by the purgingrecipe 4 is started. Supply of power to the heater 51 is stopped at timeT11 and the reaction vessel 2 is cooled in a forced cooling mode bysupplying cold air of, for example 0° C. through the air supply duct 53into the space between the reaction vessel 2 and the heating furnace 52and discharging air from the space through the discharge line 57. Thedifference between the fifth temperature and the temperature of thereaction vessel 2 at the start of the purging process is in the range ofabout 100° C. to about 500°. A cooling mode in which the reaction vessel2 is cooled rapidly at a cooling rate of, for example, 75° C./min iscalled a rapid cooling mode.

In a period between times T11 and T12, N₂ is supplied from the purginggas source 37 through the gas supply pipe 32 into the reaction vessel 2at a flow rate on the order of 1 slm, the main valve MV is closed, theneedle valve NV is opened and the vacuum pump 41 is operated todischarge the atmosphere in the reaction vessel 2 at a discharge rate onthe order of 30 l/min through the by-pass line 46. The first purgingprocess is completed at time T12. The process time of the first purgingprocess is, for example, about 12.5 min.

Then, the wafer boat 25 is loaded into the reaction vessel 2, the openlower end 21 of the reaction vessel 2 is closed by the first lid 23, andthen the second purging process is started. In FIG. 6, a mounting stepis placed before a loading step. However, since wafers W are not mountedon the wafer boat 25, the time for the mounting step is zero second. Ina period between the loading step and a waiting step, N₂ is suppliedfrom the purging gas source 37 into the reaction vessel 2 at a flow rateon the order of 1 slm. Supply of power t the heater 51 is started attime T13 to heat the interior of the reaction vessel 2 at a sixthtemperature of, for example, 750° C. by time T14. Desirably, the sixthtemperature is higher than a process temperature for the following filmforming process. The interior of the reaction vessel 2 is kept at thesixth temperature for a predetermined time. At time T15, cooling of thereaction vessel 2 is started to decrease the temperature of the interiorof the reaction vessel 2 to a temperature equal to or lower than aprocess temperature for the following film forming process.

After the wafer boat 25 has been loaded into the reaction vessel 2, theneedle valve NV is opened and the vacuum pump 41 is operated todischarge the atmosphere in the reaction vessel 2 through the by-passline 46. Then, the main valve MV is opened to discharge the atmospherein the reaction vessel 2 through the main line 45 to crate a vacuum onthe order of 0.4 Pa in the reaction vessel 2. The pressure in theevacuated reaction vessel 2 is in the range of 0.4 to 1.0 Pa.

After a predetermined vacuum has been created in the reaction vessel 2,a purging cycle using N₂ is carried out. The purging gas source 37supplies N₂ at a flow rate of 1 slm (N₂ purging step), supply of N₂ isstopped and the reaction vessel 2 is evacuated by the vacuum pump 41(VAC step). The main valve MV is opened and the vacuum pump 41 isoperated for evacuation during the N₂ purging step. Supply of N₂ isstopped after the completion of the fourth cycle of the N₂ purging step.The main valve MV and the needle valve NV are closed to stop evacuationusing the vacuum pump 41, and N₂ is supplied from the purging gas source37 at 30 slm to adjust the pressure of the interior of the reactionvessel 2 to the atmospheric pressure. Then, N₂ is supplied from thepurging gas source 37 into the reaction vessel 2 at a flow rate of 1 slmand the wafer boat 25 is unloaded from the reaction vessel 2 to completethe purging process. The process time of the purging process is, forexample, on the order of 35.2 min.

The purging recipe 4 is a basic recipe used for the film forming processthat uses BTBAS gas specified by the film forming recipe 4 as the filmforming gas. Production of particles due to production of gases in thereaction vessel 2 and the resultant separation of the deposited filmsfrom the inside surface of the reaction vessel 2 can be suppressed bycarrying out the purging process specified by the purging recipe 4 afterthe completion of every cycle of the film forming process specified bythe film forming recipe 4.

The effect of the purging process specified by the purging recipe 4 insuppressing production of gases and particles is higher than those ofthe purging processes specified by the purging recipes 1, 2 and 3.Therefore, the purging process specified by the purging recipe 4 cansuppress the production of gases and particles in the film formingprocess specified by the film forming recipes 1, 2 or 3 even if thecumulative thickness is greater than the threshold cumulative thicknessA in the film forming process specified by the film forming recipe 2,even if the cumulative thickness is greater than the second thresholdcumulative thickness A2).

The reaction vessel is cooled by force air cooling at an initial stageof the purging process specified by the purging recipe 4. As mentionedabove, when the reaction vessel 2 is cooled by forced air cooling,surface layers, which are about to come off, of films a main productsand by-products deposited on the inside surface of the reaction vessel 2come off the inside surface of the reaction vessel 2 and are dischargedoutside the reaction vessel 2 together with the atmosphere in thereaction vessel discharged through the by-pass line 46 by the vacuumpump 41.

The interior of the reaction vessel 2 is heated up to a sixthtemperature while the reaction vessel 2 is being evacuated to promotethe separation of the deposited film from the inside surface of thereaction vessel 2 and the resultant production of gases by thedifference in heat capacity between the reaction vessel 2 and thedeposited films. Consequently, the remaining films become easy toproduce gases and the residual films are forced to produce gases.Heating the reaction vessel 2 at the sixth temperature higher than aprocess temperature for the following film forming process can suppressthe production of gasses during the following film forming process.

The second purging process is carried out with the empty wafer boat 25placed in the reaction vessel 2. Thus the films deposited on the waferboat 25 can be removed to suppress production of particles and degassingof the residual films can be suppressed. The first purging process iscarried out without placing the wafer boat 25 in the reaction vessel 2.When the wafer boat 25 is unloaded from the reaction vessel 2, the waferboat can be cooled in a natural cooling mode. The effect of the naturalcooling mode is equal to or higher than the forced air cooling mode.Consequently, the separation of the deposited films can be promoted.

Operations of a series of processes, namely, the film forming processspecified by a predetermined film forming recipe and the purging processspecified by the predetermined purging recipe, are controlled by thecontroller 7 on the basis of the software stored in the controller 7.

The inventors of the present invention made studies and found thatsurface parts, which produces gases and particles, of films deposited onthe inside surface of a reaction vessel can be removed and thereby theproduction of gases and particles in the following film forming processcan be suppressed by carrying out a purging process having a purgingeffect compatible with a film forming process specified by a filmforming recipe. The present invention has been made on the basis of thefindings found through the studies. The inventors of the presentinvention analyzed causes of production of particles in each filmforming process, produced purging recipes specifying purging processesrespectively compatible with forming processes specified by film formingrecipes through a trial-and-error method to accomplish the presentinvention.

The present invention prepares purging recipes specifying purgingprocesses having effects compatible with film forming processesspecified by film forming recipes, respectively, selects automatically apurging recipe specifying a purging process compatible with a filmforming process when each cycle of the film forming process iscompleted, and processes the reaction vessel by the purging processspecified by the selected purging recipe. Some of the film formingprocesses require a purging process having a strong purging effect andthe others require a purging process having a moderate purging effect.The process time of the purging process having a strong purging effectis long.

More concretely, the present invention prepares the four purging recipesrespectively specifying purging processes respectively having differentpurging effects, namely, the purging recipe 1 specifying the purgingprocess having the shortest process time of 12.5 min and not causing anydowntime, the purging recipe 2 specifying the purging process having aprocess time of about 30.7 min, causing a downtime equal to the processtime and having a purging effect higher than that of the purging processspecified by the purging recipe 1, the purging recipe 3 specifying thepurging process having a process time of about 34.4 min, causing adowntime equal to the process time and having a purging effect higherthan those of the purging processes specified by the purging recipes 1and 2, and the purging recipe 4 specifying the purging process having aprocess time of 47.7 min causing a downtime of about 35.2 min and havinga purging effect higher than those of the purging processes specified bythe purging recipes 1, 2 and 3.

The purging process having a purging effect compatible with the filmforming process can remove the films that are deposited on the insidesurface of the reaction vessel and produce gases and particles, avoidingspending time on useless purging process, such as spending the longprocess time of the purging process specified by the purging recipe 4after the film forming process using DCS gas as a film forming gas. Thusdowntime during which the film forming system is not operating to carryout the film forming process can be removed or reduce to the shortestpossible time and, consequently, the reduction of through put due to thepurging process can be suppressed.

The inventors of the present invention found that different cumulativethicknesses of the films deposited on the inside surface of the reactionvessel 2, different operating times of the reaction vessel 2 anddifferent numbers of particles in the loading area 28 and the dischargepipe 43 require different purging recipes suitable for suppressing theproduction of gases and particles. In such a case, the purging recipe ischanged automatically for a suitable purging recipe compatible with thecondition of the reaction vessel 2. Consequently, the films deposited onthe inside surface of the reaction vessel and causing the production ofgases and particles can be surely removed after the every cycle of thefilm forming process.

More concretely, the film deposited on the inside surface of thereaction vessel 2 is liable to produce gases, to crack and to come offwhen the cumulative thickness of the films deposited on the insidesurface of the reaction vessel 2 is greater than the thresholdcumulative thickness A. Therefore, the purging process specified by thepurging recipe 4 and having the highest purging effect is carried out toremove the film deposited on the inside surface of the reaction vessel 2completely when the cumulative thickness is greater than the thresholdcumulative thickness A. When the cumulative thickness increasesexcessively, the films deposited on the inside surface of the reactionvessel 2 become easy to come off. Therefore, in such a case, the purgingprocess specified by the purging recipe 4 and having the highest purgingeffect is carried out when the number of particles in the loading area28 or the discharge pipe 43 increases suddenly.

The amount of gases produced by the deposited films increase, thedeposited films on the inside surface of the quartz reaction vessel 2crack and particles are produced when the cumulative operating timeincreases excessively. In such a case, a purging process having apurging effect higher than that of the basic purging process specifiedby the basic purging recipe may be carried out.

It is effective to select a purging recipe other than the basic purgingrecipe on the basis of the cumulative thickness of the films formed inthe reaction vessel 2, the cumulative operating time of the reactionvessel 2, the number of particles in the discharge pipe 43 and thenumber of particles adhering to the surfaces of the wafers W and tocarry out a purging process specified by the selected purging recipe.However, such a purging recipe does not need necessarily to be selectedand a purging process specified by a basic purging recipe compatiblewith the film forming recipe may be carried out. A purging recipe otherthan the basic purging recipe may be selected on the basis of one or twoof conditions including the cumulative thickness of the films formed inthe reaction vessel 2, the cumulative operating time of the reactionvessel 2, the number of particles in the discharge pipe 43 and thenumber of particles adhering to the surfaces of the wafers W. and apurging process specified by the selected purging recipe may be carriedout. Particles in the reaction vessel 2 may be measured in addition tothose in the discharge pipe 43. Particles adhering to the surfaces ofthe wafers W may be measured either in or outside the reaction vessel 2.

The present invention is applicable not only to the film forming processfor forming SiN films, but also to film forming processes respectivelyfor forming TaN films, TiN films and WN films. The film forming processis not limited to a film forming process for forming films of the sametype and may be a film forming process for forming films of differenttypes.

Degassing occurs when HCD gas is used as a film forming gas, the filmforming process specified by the film forming recipe 2 specifying aprocess temperature below 500° C., such as 450° C. and the film formingprocess specified by the film forming recipe 3 specifying a processtemperature not lower than 500° C., such as 600° C. are carried outsuccessively, and the film forming process specified by the film formingrecipe 3 is carried out after the purging process specified by the basicpurging recipe 2 compatible with the film forming recipe 2 even if thecumulative thickness after the film forming process specified by thefilm forming recipe 2 is smaller than the threshold cumulative thicknessA1. Therefore, in such a case, it is preferable to carry out the purgingprocess specified by the purging recipe 3 or 4 having a purging effecthigher than that of the purging process specified by the purging recipe2.

A second embodiment of the present invention will be described. Thesecond embodiment is analogous with the first embodiment, except thatthe second embodiment uses NH₃ gas instead of N₂ as a purging gas. Thedescription of the second embodiment will be made with reference toFIGS. 1 to 12, in which N₂, namely, the purging gas, is replaced withNH₃ gas, and NH₃ gas, namely, a purging gas, is supplied from the filmforming gas source 36 that supplies NH₃ gas as a film forming gas.

The film forming process specified by the film forming recipe 1 forms aSiN film on the surface of the wafer W by using DCS gas and NH₃ gas. Thefilm forming processes specified by the film forming recipes 2 and 3form a SiN film on the surface of the wafer W by using HCD gas and NH₃gas. Therefore, in the film forming processes for forming the SiN film,Cl atoms contained in the DCS gas and the HCD gas penetrate into the SiNfilm. Consequently, surface parts of the SiN films deposited on theinside surface of the reaction vessel 2 and the wafer boat 25 emit Clatoms when the surface parts of the SiN films are caused to come off dueto the forced air cooling or heating of the reaction vessel 2 during thepurging process specified by, for example, the purging recipe 1 or 3.The Cl atoms emitted by the surface parts of the deposited SiN filmsadhere to the inside surface of the reaction vessel 2 and the wafer boat25. The Cl atoms adhering to the inside surface of the reaction vessel 2and the wafer boat 25 scatter and adhere to the surfaces of wafers Wwhen the wafer boat 25 holding the wafers W is loaded into the reactionvessel 2 and the next film forming process is carried out. When SiNfilms are formed on the surfaces of the wafers W, respectively, byusing, for example, DCS gas and NH₃ gas after thus loading the waferboat 25 into the reaction vessel 2, the Cl atoms adhering to thesurfaces of the wafers W delay the formation of the SiN films by anincubation time; any films are not formed upon the supply of the filmforming gases onto the surfaces of the wafers W and the film startsrapidly growing on the surfaces of the wafers W after a certain time haselapsed since the supply of the film forming gases onto the surfaces ofthe wafers W. Consequently, the SiN films thus formed have irregular,rough surfaces and irregular thickness. Such SiN films are inferior inelectrical characteristics and etch characteristics.

When NH₃ gas is used as a purging gas instead of N₂ in the purgingprocesses (cycle purging) specified by the purging recipes 1 to 4, theCl atoms contained in the surface parts of the SiN films and NH₃ gasinteract and NH₄Cl (ammonium chloride) is produced. Then, NH₄Cl isdischarged from the reaction vessel 2.

This film forming method has an effect of preventing the deteriorationof, for example, the electrical and etching characteristics of the SiNfilms formed on the surfaces of wafers W after the reaction vessel 2 hasbeen processed by the purging process in addition to those of the filmforming method using N₂ as a purging gas.

The purging method using NH₃ gas as a purging gas is applicable not onlyto a method that selects a purging recipe automatically on the basis ofthe cumulative thickness of films formed by a film forming processspecified by a selected film forming recipe, but also to a method thatpurges the empty reaction vessel 2 not holding any wafers therein. Thispurging method can be applied to, for example, a method that purges thereaction vessel 2 by a purging process after the wafer boat 25 holdingprocessed wafers processed by a film forming process has been unloadedfrom the reaction vessel 2 and the reaction vessel has been sealed, suchas a method that processes the reaction vessel 2 by a purging processthat supplies NH₃ gas as a purging gas into the reaction vessel 2 whilethe reaction vessel 2 is being cooled by forced cooling, and a methodthat cools the reaction vessel 2 loaded with or not loaded with thewafer boat 25 by forced cooling or heating the interior of the reactionvessel 2 at a temperature higher than the process temperature of thefilm forming process by rapid heating and processes the reaction vesselby a purging process. The purging gas is not limited to pure NH₃ gas.The purging gas may be a mixed gas prepared by mixing NH₃ gas and adiluting gas, such as N₂.

This purging method is effective when a film forming gas containingchlorine, such as a chlorine compound gas or chlorine gas, is used as afilm forming gas. Therefore, the purging method is applicable to apurging process that is to be carried out after a Ti film formingprocess using, for example, TiCl₄ gas (titanium tetrachloride gas).

A third embodiment of the present invention will be described. The thirdembodiment is analogous with the first embodiment, except that the thirdembodiment uses Oxygen gas (O₂) or a mixed gas prepared by mixing O₂ andan inert gas, such as N₂ instead of N₂ as a purging gas. The descriptionof the second embodiment will be made with reference to FIGS. 1 to 12,in which N₂, namely, the purging gas, is replaced with O₂, and O₂,namely, a purging gas, is supplied from an O₂ source, not shown.

SiN films adhering to the inside surface of the reaction vessel 2 andthe wafer boat 25 are removed by cooling the reaction vessel 2 by forcedcooling or heating the reaction vessel by the purging process specifiedby the purging recipe 1 or 3. The adhesion of SiN films to quartz isstrong and, therefore, the SiN films adhering to the inside surface ofthe reaction vessel 2 made of quartz and the wafer boat 25 made ofquartz are difficult to remove and the inside surface of the reactionvessel 2 made of quartz and the surface of the wafer boat 25 made ofquartz are cracked when the SiN films are separated from them. It isinferred that the inside surface of the reaction vessel 2 and thesurface of the wafer boat 25 are cracked because a high tensile stressis induced in the inside surface of the reaction vessel 2 and the waferboat 25 when the SiN films are removed from the inside surface of thereaction vessel 2 and the wafer boat 25.

The purging processes (cycle purging) specified by the purging recipes 1to 4 use, for example, O₂ as a purging gas instead of N₂. Thus a gascontaining oxygen is used for purging the reaction vessel 2 to form aSiO₂ film on the SiN film to induce a compressive stress in the surfacesof the SiN film. Consequently, the tensile stress induced in the SiNfilm is reduced to prevent the separation of the residual, unstable SiNfilm remaining o the inside surface of the reaction vessel 2 and thesurface of the wafer boat 25.

The third embodiment has an effect of oxidizing the unstable, residualSiN film remaining on the surfaces of the quartz members to prevent theunstable, residual SiN film from coming off the surfaces of the quartzmembers, to suppress the contamination of the wafers with the unstableSiN film that falls off the surfaces of the quartz members during thefilm forming process and to suppress the development of cracks in thesurfaces of the quartz members in addition to the effect of the firstembodiment using N₂ as a purging gas.

1. A film forming method of processing substrates by a film formingprocess using film forming gases by a thermal processing system providedwith a plurality of film forming recipes, said film forming methodcomprising the steps of: processing substrates held on a substrateholder placed in a reaction vessel by a film forming process specifiedby a selected one of the film forming recipes; unloading the substratesfrom the reaction vessel; automatically selecting one of a plurality ofpreviously prepared purging recipes on the basis of a cumulativethickness equal to the sum of thicknesses of thin films formed by cyclesof the film forming process specified by the selected film formingrecipe; and carrying out a purging process specified by the selectedpurging recipe to purge the reaction vessel by supplying a purging gasinto the reaction vessel after the substrates have been unloaded fromthe reaction vessel.
 2. The film forming method according to claim 1,wherein the plurality of film forming recipes include those respectivelyspecifying different film forming gases to form thin films of the sametype.
 3. The film forming method according to claim 1, wherein theplurality of film forming recipes include those respectively specifyingthe same film forming gases and different process temperature ranges. 4.The film forming method according to claim 1, wherein the plurality ofpurging recipes include those differing from each other in at leasteither of purging time and temperature condition in the reaction vessel.5. The film forming method according to claim 1, wherein the substrateholder holds a plurality of substrates in parallel layers, and thepurging process includes a first purging process that is carried out ina state where the reaction vessel is sealed after the substrate holderhas been unloaded from the reaction vessel.
 6. The film forming methodaccording to claim 1, wherein the substrate holder holds a plurality ofsubstrates in parallel layers, and the purging process includes a secondpurging process that is carried out in a state where the empty substrateholder is placed in the sealed reaction vessel.
 7. The film formingmethod according to claim 1 further comprising the step of changing thepurging recipe on the basis of at least either of the measured number ofparticles in the reaction vessel and the measured number of particlesadhering to the surfaces of the substrates.
 8. The film forming methodaccording to claim 1, wherein the first and/or the second pursingprocess includes the step of forcibly cooling the reaction vessel. 9.The film forming method according to claim 1, wherein the second purgingprocess includes the step of heating the reaction vessel at atemperature higher than a process temperature for a film forming processfollowing the purging process.